1. Field of the Invention
The present invention relates to tubular threaded connections, and joints or assemblies of tubes to be joined by threads.
2. Discussion of the Background
The tubes described herein are used in industry and, in particular, assemblies or threaded junctions used in string-lines for tubing or for lines of tubular production accessories or for a casing or a liner or a riser for the operation or prospecting or exploitation of oil or gas wells. The threaded assemblies or junctions described herein may also be used for any purpose where it may be desired to assemble pipelines or tubular accessories such as, for example, in geothermal or steam plants. The threaded assembly described herein is particularly useful in the assembly of metal tubes used for the casing of oil or gas wells or for so-called liners beyond the bottom of the casing string, as explained below.
Numerous types of assemblies are known for petroleum or gas carrying tubes that yield satisfactory results from the viewpoint of mechanical characteristics and tightness, even under tough conditions of use. Certain of these assemblies involve the use of tubes equipped with male frustoconical threads at both ends, which are assembled by couplings having two corresponding female frustoconical threads. This manner of assembly offers the advantage of rendering the two components of the assembly rigid, due to the existence of the positive interference that one can create between the male threads and the female threads.
However, the external diameter of these couplings is greater than that of the corresponding tubes and, when these assemblies are used with casing tubes, the couplings require that bore holes with increased diameter be drilled to accommodate the external diameter of the couplings. In the case of very deep wells, with a depth exceeding 4,000 meters, the initial diameter of the well first casing strings and consequently the diameter of the well in the vicinity of the surface may be twice as large using these couplings as it could be using slim couplings having an external diameter only slightly larger than the corresponding tubes of the casing strings.
To prevent this difficulty, one may use assemblies without a coupling or sleeve. In this case, the tubular elements each have one male threaded end and one female threaded end, making for a thin assembly. These assemblies or junctions are generally referred to as integral assemblies or junctions, in contrast to assemblies or junctions employing a coupling or sleeve. The same need for integral junctions is also met in the case of liners which are hung on the casing string at the bottom thereof, are not cemented to the borehole and often run horizontally to reach the oil or gas producing formation. In particular, exploitation of non-conventional gas reservoirs, such as so-called shale gas reservoirs, require such small diameter and slim liners with integral junctions.
Integral assemblies are generally made on tubes which include an expanded diameter at the end corresponding to the female threads, and a reduced diameter at the end corresponding to the male threads. This is done in order to have sufficient material in the thickness of the tubes to ensure the geometric and mechanical strength of the assembly that joins the tubes.
It is also possible to reinforce the strength of the male-female assembly by using threads in two successive sections, or steps, instead of only a single one. Each of the steps of threads has different thread diameters and is separated by a central ring-shaped abutment. This abutment makes it possible to achieve sufficient tightening of the threads while at the same time avoiding excessive on-screwing. In the case of threads with negative load flanks, the abutment makes it possible to tighten these threads on their negative flanks and this reduces risks of thread disengagement due to the action of traction stresses that may or may not be combined with strong pressures. However, an abutment between steps of threads requires allocation of radial material thickness within the junction assembly, thereby increasing the overall outer diameter of the assembly at the junction.